Transmural variation in the length‐force relationships and in vivo operating length ranges of the mantle muscles of squid

Abstract

Recent work has shown that contraction of hollow, cylindrical, muscular organs is accompanied by significant non‐uniformities in circumferential strain across the muscular body wall. In squid, circumferential strain along the inner surface of the mantle wall is 1.3 to 1.6 times greater during the mantle contractions used to power jet locomotion than along the outer surface of the mantle. This transmural gradient of strain may result in the circular muscle fibers from near the inner and outer surfaces of the mantle operating over different regions of the length‐tension curve during the same mantle contraction cycle, potentially generating different instantaneous forces. We used sonomicrometry on swimming squid and isometric tests on bundles of circular fibers to test the hypothesis that circular muscle contractile properties vary transmurally in the mantle in the Atlantic longfin squid, Doryteuthis pealeii. We found that the length‐force relationship of the obliquely striated circular muscle fibers varied significantly with radial position in the mantle wall. Sonomicrometry experiments revealed that the circular fibers operated in vivo primarily along the entire ascending limb of the length‐tension curve. Our work shows that the functional diversity of obliquely striated muscles is much greater than previously thought. Supported by NSF grant IOS‐0950827.